DC voltage converters are used in portable devices in which the batteries provide only a low supply voltage on account of the desired miniaturization and weight saving. In order to supply the circuit units of the devices, a DC voltage converter converts the supply voltage into a higher output voltage. The design of such a DC voltage converter (which is also referred to as a step-up converter) is described in Tietze/Schenk: “Halbleiterschaltungstechnik” [Semiconductor circuit technology], 12th edition, pages 948 to 949.
A DC voltage converter comprises an inductive storage element, which is connected between a supply voltage terminal and, in such a manner that it is coupled via a first switch, a reference potential terminal. The capacitively buffered terminal for the output voltage is connected, via a second switch, between the inductive storage element and the first switch.
During ideal cyclic operation, the first switch and the second switch change over simultaneously. If the first switch is on and the second switch is off, energy is stored in the inductive storage element. The charge is removed if the second switch is on and the first switch is off, and the capacitor is charged. If the switching states of the first and second switches remain unchanged, the coil current that drains from the inductive storage element falls continuously until the inductive storage element has been discharged and no more coil current flows. This operating mode is also referred to as intermittent operation. If the first switch and the second switch are changed over before the coil current has ceased, this operating mode is referred to as continuous operation.
So that the output voltage assumes a prescribed value, provision is made of a regulating circuit, which is coupled, on the output side, to the control inputs of the first and second switches and regulates the switching states of the latter. The regulating circuit is configured in such a manner that it operates in one of two operating modes depending on whether the DC voltage converter is operating intermittently or continuously.
Use has been made of DC voltage converters which operate either intermittently or continuously, with the result that the regulating circuit is designed for only one operating mode.
In the case of regulating circuits which can be changed over between the two operating modes, the operating mode of the DC voltage converter must be determined so that the regulating circuit operates in the corresponding operating mode. In some instances the coil current has been measured in order to distinguish between the two operating modes of the DC voltage converter. This is associated with a considerable outlay in circuitry.